It would be cool to have a notebook in pix pro for quickly get to information. I cant always remember how big the pics most be for the web if you have different columns. Would be great to paste the values in there and quickly access it maybe a button in the taskbar...

It very much depends, a feature like this wouldn't really affect performance at all. Things that somehow change your image have the potential to affect performance. Say, if you apply a blur effect live to an image, that can slow things down (not with modern computers, of course). But then the programmers' job is to optimize things using all kinds of programming tricks to avoid that. Say you have a big image with all kinds of layer with adjustments, layer styles, and effects applied nondestructively (i.e. the original images and the data about the changes has to be saved somewhere) — that can also slow things down. But, again, the programmers have to create an architecture that allows for everything to be saved and accessed quickly and efficiently. Pixelmator Pro is already quick but we actually have plans to optimize lots of things even more, so it should only get faster from here on out.